The Sources folder

When people work on "a project" - meaning a program made to accomplish some task - there will often be more than just Java code involved.

To deal with this its common to make a folder to put Java code into. I prefer the name src for this.

$ mkdir src

And inside this src folder we will put the code.1

So you should have this

project/ <- Open this in your text editor
  src/ <- Code will go here
1

There are different ways to layout a project. All are valid. You can call the src folder STUFF - it ultimately doesn't matter. This is just another one of those social conventions.